Ventless delay electric initiators



Nov. 20, 1956 l. o. LEWIS ETA!- VENTLESS DELAY ELECTRIC INITIATORS Filed Aug. 22, 1952 United States Patent 2,771,033 VENTLESS DELAY ELECTRIC INITIATO Idwal 0. Lewis, Ardrowwan, and Robert D. J. Owens, Saltcoats, Scotland, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain Application August 22, 1952, Serial No. 305,906 Claims priority, application Great Britain November 22, 1951 3 Claims. (Cl. 102-28) The present invention is concerned with improvements in or relating to delay electric initiators and is particularly concerned with ventless short delay electric detonators.

The object of the present invention is to provide series of ventless short delay electric detonators having delay intervals between successive delay numbers for example of the order 25 or 50 milliseconds which, when the ventless container consists at least essentially of copper, have the advantage that single detonators of any such practical series when fired freely suspended in an inflammable gas mixture, for example, 9% methane/ air mixture, show no tendency to cause ignition of the gas mixture, and which when the ventless container is of aluminium or copper or copper alloys are cheaper to produce than has heretofore been possible.

According to the present invention the ventless short delay electric detonator comprising an electrically actuated igniting charge, an initiating charge, and an interposed short delay element enclosed in a ventless container is characterised in that the said short delay element comprises a cut-ofi length of fuse comprising a flexible envelope surrounding a core comprising a preferably graphite-glazed, nitrocellulose bonded granular rmxture of silicon and red lead.

Preferably the ventless container consists essentially of copper.

The bonded granular mixture of the core of the delay fuse element may contain red lead in an amount from 65 to 95 percent by weight, silicon in amount from to 35 percent by Weight and nitrocellulose in amount up to 5 percent by weight of the red lead plus silicon. It is preferred, however, generally for the compositions to consist of 70 parts red lead, 30 parts silicon and 0.25 part nitrocellulose by weight.

These mixtures are preferably made by first dry nnxing the red lead and silicon, then mixing the resulting mixture with a solution of nitrocellulose in a suitable solvent, for example, amyl acetate, in sufiicient amount to give a dry crumbly mass which is then sieved, for example, through a 25 B. S. S. mesh sieve and dried to give the bonded granular mixture.

The bonded granular mixture is preferably glazed by tumbling with graphite in a drum or other suitable container, for example, for a time of the order of eight hours to give a final composition containing less than 1 percent of graphite. The step of glazing enables the granular compositions to be incorporated into fuse more easily than the same compositions unglazed.

The flexible envelope may be formed of glass or textile yarns and may be provided with an outer coating which may consist of an organic thermoplastic material such as, for example, polyethylene, polyvinyl chloride or polystyrene.

The textile covering may be spun around a core of the glazed material and compressed against it by passage through a die and the fuse thereafter finished in the manner known in the manufacture of safety fuse.

Delay elements of differing delay periods can be obtained either by using different lengths containing the same composition or by varying the composition.

It is also preferred that the delay element be secured in position by pressing an inverted perforated metal cup over the end thereof remote from the initiating composi- I 2,711,033 Patented Nov. 20, 1956 'ice tion, instead of relying on a friction fit between the outer envelope and the wall of the tubular container. The use of such a cup also prevents the envelope, if ignitible, from being ignited by the igniting charge. In inserting the delay elements of the invention into the tubular containers the soft and somewhat compressible nature of the envelope renders the attainment of the necessary tight fit easy without the application of loading pressures so high as to damage the structure of the delay element. An expansion space should be left between the end of the delay element and the igniting charge. If considered desirable or necessary a small supplementary charge of, preferably gasless, igniting composition may be interposed between the end of the delay element and the initiating charge. It is also sometimes desirable to press a small charge of a gasless igniting composition directly on top of the delay element in order to ensure its ignition from the electrically actuated igniting charge, which will usually be spaced from it. The electrically actuated igniting charge may take the form of an electric fusehead.

One embodiment of the invention is illustrated by the accompanying diagrammatic drawing, in which 1 is an aluminium detonator tube, 2 is a detonating explosive charge of tetryl compressed into the end of the tube, 3 is a more sensitive detonating charge compressed on top of charge 2 and consisting essentially of a mixture of aluminium, lead azide and lead 'styphnate and constituting the initiating charge, 4 is the flexible envelope, 5 is a covering of polyethylene and 6 is a core of a glazed bonded granular mixture. 7 and 8 are layers of gasless igniting composition consisting of silicon and red lead in the weight ratio 30 to 70, 9 is an inverted perforated metal cup pressed over the composition 8 and over the top of the delay element so as to secure the delay element longitudinally in the tube, 10 is an electric fusehead, 11 is an axially perforated sealing plug of rubber-like material crimped between the aluminium tube 1 and the insulated electric leading wires 12 and 12' as shown at 13 whereby there is attained a seal capable of withstanding an internal pressure greater than the maximum pressure that can be produced as a result of the combustion of the contents of the detonator tube with the exception of the detonating explosive charges 2 and 3. The space 14 between the end of the plug 11 and the inverted cup 9 and hence the length of the tube 1 is accordingly made sufficient to ensure that the maximum pressure that can even temporarily be developed in the initiator by the successive consumption of the compositions of the electric fusehead 10, the igniting charge 8, the delay composition 6, and the igniting charge 7 will not unseal the tube.

The invention is also illustrated by the following examples in Table 1 in which are recorded lengths of delay elements, mean delay times, and the number of ignitions given when single detonators are fired freely suspended in a 9 percent methane/ air mixture. The delay composition consists of 70 parts red lead, 30 parts silicon and 0.25 nitrocellulose. The detonators are commercial lead azide/lead styphnate/aluminium/tetryl delay detonators in copper tubes without any supplementary igniting charges top and bottom.

Table 1 Length of Size of delay Mean delay No. of Example detonator element time (mil1llgnitlons/ (N 0.) (inches) seconds) No. tested 6 1/8 30 0/10 6 0/10 6 1% 250 cm 6 0. 2 35 o/25 6 0.5 100 43/25 6 1.0 280 0/65 8 0.6 0/60 8 1.0 220 0/40 s 1. 2 260 0/25 In Exmples 13 the delay composition is not glazed with graphite and in Examples 4-9 the delay composition is glazed with graphite after the bonding with nitrocellulose. v

The invention is further illustrated by the following examples in Tables 2 and 3 in which are recorded delay periods of delay fuse elements ofthe lengths shown made from fuse whose core consists of a nitrocellulose bonded granular mixture having the composition given when used in commercial lead azide/lead styphnate/aluminium delay detonators without any supplementary igniting charges top and bottom.

Table 2 Fuse core composition Mean Fuse delay Example Length, time,

Red Silicon Nitroinches millilead cellulose seconds Table 3 Delay times in milliseconds Example Fuse length, inches Mean Max. Min

4 The fuse composition for Examples 4 to 9 is the same as that in Example 3.

What we claim is:

1. A ventless short delay electric detonator comprising an electrically actuated igniting charge, an initiating charge and an interposed short delay element enclosed in a ventless container essentially of copper wherein the said short delay element comprises a cut-ofi length of fuse comprising a flexible textile envelope around a core comprising a dry crumbly graphite-glazednitrocellulose bonded granular mixture of silicon and red lead, said red lead being in amount of from about 65% to 95% by weight, the silicon in amount of from about 5% to 35% by weight, and the nitrocellulose in amount up to about 5% by weight of the red lead andsilicon.

2. A ventless short delay electric detonator as claimed in claim 1 wherein the bonded granular mixture consists of 70 parts red lead, parts silicon, and 0.25 part nitrocellulose byweight.

3. A ventless short delay electric detonator as claimed in claim 1 wherein the fiexilble envelope has an outer coating of an organic thermoplastic material.

References Cited in the file of this patent UNITED STATES PATENTS 1,825,224 Ellsworth Sept. 29, 1931 1,999,820 Nash Apr. 30, 1935 2,079,777 Taylor et al May 11, 1937 2,410,801 Audrieth Nov. 12, 1946 2,416,639 Pearsall Feb. 25, 1947 2,478,501 Patterson Aug. 9, 1949 2,513,391 Zenftman July 4, 1950 2,586,959 Kerr et al Feb. 26, 1952 2,619,035 Lewis et al. Nov. 25, 1952 

1. A VENTLESS SHORT DELAY ELECTRIC DETONATOR COMPRISING AN ELECTRICALLY ACTUATED IGNITING CHRAGE, AN INITIATING CHARGE AND AN INTERPOSED SHORT DELAY ELEMENT ENCLOSED IN A VENTLESS CONTAINER ESSENTIALLY OF COPPER WHEREIN THE SAID SHORT DELAY ELEMENT COMPRISES A CUT-OFF LENGTH OF FUSE COMPRISING A FLEXIBLE TEXTILE ENVELOPE AROUND A CORE COMPRISING A DRY CRUMBLY GRAPHITE-GLAZED NITROCELLULOSE BONDED GRANULAR MIXTURE OF SILICON AND RED LEAD, SAID RED LEAD BEING IN AMOUNT OF FROM ABOUT 65% TO 95% BY WEIGHT, THE SILICON IN AMOUNT OF FROM ABOUT 5% TO 35% BY WEIGHT, AND THE NITROCELLULOSE IN AMOUNT UP TO ABOUT 5% BY WEIGHT OF THE RED LEAD AND SILICON. 